I don't understand how to solve problems using number sense. For
example, if an employee at the deli counter slices a 2 foot long
salami, about how many slices will he get? How can I figure that out?
Why is it important?

The justification that 0 is an even number is based on the fact that 0 is
divisible by 2. But how many times does 2 go into 0? Answer: no times; or
to rephrase, 2 doesn't go into 0. Can you explain this?